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Mercedes-Benz Presents the Combined SI-CAI “DiesOtto” Concept Engine

Mercedes-Benz provided some details on its DiesOtto engine project: a 1.8-liter, turbocharged, four-cylinder gasoline direct injection engine that supports a switch to controlled auto ignition (CAI) under certain operating conditions.

When starting and under full load, the fuel/air charge is ignited by a spark plug, as in a conventional spark-ignition (SI) engine. The switch to controlled auto ignition mode occurs under partial load conditions, i.e. at low and medium engine speeds.

Despite its reduced displacement—downsizing being one of the major factors for reducing fuel consumption —the research engine delivers an output of 175 kW/238 hp and a maximum torque of 400 Nm (295 lb-ft), with projected fuel consumption of less than 6.0 l/100km (39 mpg US). This, Mercedes-Benz said, could power a vehicle the size of its current S-Class.

Our next goal will now be to make the gasoline engine as economical as a diesel. All the preconditions for this are provided by our DiesOtto concept, which incorporates the foremost strengths of both the gasoline engine and diesel engine.

—Prof. Dr. Herbert Kohler, Head of Group Research & Advanced Engineering Vehicle and Powertrain; Chief Environmental Officer of DaimlerChrysler

Volkswagen has also earlier seized on the term “DiesOtto” to reflect its Combined Combustion System (CCS) work (earlier post). A number of automakers, engineering firms and universities are exploring ways to combine CAI/HCCI with conventional combustion systems as a way to increase the fuel economy of gasoline-fueled engines. (Earlier post.)

Some of the issues involved in engineering such combined systems include the design of control systems for cycle-to-cycle control, ways to enlarge the operating range for the CAI mode, and transient response.

Features of the Mercedes-Benz DiesOtto engine include:

  • Downsizing with fewer cylinders and a smaller displacement;

  • Turbocharging for enhanced performance;

  • Direct gasoline injection;

  • Controlled auto ignition;

  • Variable valve control;

  • A variable compression ratio; and

  • An optional microhybrid module with an integrated starter/generator.

The low temperature combustion reduces engine-out NOx. All further emissions control in the DiesOtto engine is by means of a standard three-way catalytic converter.

Mercedes-Benz noted that some of the technologies in the concept engine—such as gasoline direct injection—are already in production, while others will emerge in the mid-term. The company again emphasized that it is working on hybrid options for its models.

(A hat-tip to Rafael!)



Bud Johns

Well, talk about using ALL of today's technology.......when they said 39 mpg it must have been in a larger car, I would think it could do better than that. 295 ft lb of torque is huge for this size engine.

Rafael Seidl

@ Bud Johns -

For reference, the smallest gasoline-powered variant of the S-Class as sold in Germany today is called the S350. It features a 3.5L V6 rated at 200kW/350Nm that gets 10.2 L/100km.

With the microhybrid masking the turbo lag to some extent, a DiesOtto would be comparable in daily driving where low-end torque matters most and, improve fuel economy by a whopping 40%. People don't drive S-Classes to hoon around, so they'll never even notice that the rated power rated power is 12% lower. Indeed, anyone looking at a DiesOtto will most likely be cross-shopping it against a comparable conventional diesel, if one is available.

The real problem with downsizing is one of consumer perception: marketing departments have spent decades convicing customers that more displacement and more cilinders equals more power and more prestige. Saving fuel first and foremost implies pinching pennies. S-Class owners are not misers, though many are businesspeople who didn't get rich by throwing money down the drain. Still, once this technology does go into production, expect the sales pitch to focus on reduced CO2 emissions and/or improving energy security, depending on the customer's political persuasion.


Forget the hybrid, just concentrate on getting this beauty finished, I would chose a DiesOtto over a hybrid model regardless of cost.

I wonder if the lofty outputs will make it to a production model. Very impressive.


"I would think it could do better than that. 295 ft lb of torque is huge for this size engine."

Really? That's a 1.8 liter engine that is within 5ft/lbs of the new BMW 3.0 Twin Turbo with GDI. That sounds pretty good to me.

Bob Bastard

I wish the 5.7L V8 in my truck performed like that.


Rafeal has some cute throw away lines but not much substance.

Any engine running two distinct combustion modes is not a smart idea. I find these outrageous claims of achieving Diesel efficiency nonsense. The fact is that this engine can neither run as lean and at as high a CR as a Diesel engine for as much of the speed and load range.

As Rafeal knows only too well Germans regularly drive at full load on the autobahns so this concept is still not as good as the current Diesels, rather much worse for real world autobahn fuel consumption.

In any event this will make a good display at the IAA and have many people believing that it is the next big deal in engine technology. I pity the poor devils who have to bring this to serial production...

Rafael Seidl

@ extracomment -

"Any engine running two distinct combustion modes is not a smart idea." On what do you base this claim? HCCI in part load adds cost but it does not take anything away from the conventional SI mode, which supports higher RPM than the diesel fuel used in conventional CI can.

Even at 200kph (125mph), you "only" need approx. 200hp to overcome wind resistance. It's only when you decide you need to accelerate even higher that you actually need to redline an engine rated at well above 200hp. As you approach rated power, the primary fuel economy advantage of a conventional diesel - the lack of a butterfly valve - becomes a moot point. The higher compression ratio does still help some and so does the fact that diesel fuel contains ~10% more energy by volume. In terms of fuel mass flow, there is little difference between an SI and a CI engine at these high power levels.

However, in combined cycle driving a DiesOtto will get close to the fuel economy of a comparable conventional CI engine without the expensive exhaust gas aftertreatment requried for T2B5/LEV II and the future Euro 6 level. That, combined with the need to match the demand mix for fuels with what refineries can produce at reasonable cost, is why there will be a place for the DiesOtto alongside the conventional diesel.

Roger Pham


here's some extracomments for you:
GDI (with modestly higher compression ratio) can run lean and powerful due to stratified-charge arrangement. Remember that at a given compression ratio, Otto cycle (isochoric combustion) is significantly more efficient than Diesel cycle (isobaric combustion). A GDI SI does not need to have as high a compression ratio as a Diesel in order to attain the same thermal efficiency.

In fact, the high compression ratio of the Diesel increases engine friction at part load in comparison to an Otto cycle engine at part load. HCCI at part load gives you dual benefit of Diesel efficiency without associated Diesel's costly disadvantage. This is a very impressive development!

If you don't much care for all the extra power of this Mercedes engine, you can go the Ford approach in the previous HCCI article and use low-cost port fuel injection and conventional 3-way exhaust catalytic converter (instead of more expensive Blue-tec to reduce NOx and particle filter for PM in Diesel and for GDI engine operating in lean stratified charge mode).


With the near(?) future coming out parties for Electric Vehicles(EVS) with viable increasing electric storage density, ICE must produce lower emissions & higher MPG. However, ICE still loses in so many ways to EVs. Tho my old Ford Festiva gives double the average MPG of ICE vehicles, EVs will still be superior. My next car will be a full EV(maybe I'll convert my Festiva to EV). ICE manufacturers had their chance to care about pollution their engines poured into the planet, but they didn't produce engines to take care of the planet. Too late to get me excited about efficient ICE engines now.

Mad Max

I wonder if this technology could be integrated with ethanol injection to increase efficiency further

Bud Johns

Litesong, let me just say that I've driven a Prius since Jan 7 '04. That being said, major advances in ICEs can only help in every respect. PHEVs need an ICE or some source of power too. All THAT being said, everybody look out for GEN III Prius. I expect it to shock everyone right to the shorts.......just a prediction, don't have privy to inside info.

Bob Bastard

HCCI technology has actually made it into street legal vehicles before. Honda has played with it in 2-cycle street/trail bikes. They call it "Activated Radical combustion," and like the Mercedes design, it switches between SI and autoignition. I believe it was used in the exp-2 as well as the NX400, and the CRM250AR which was/is a production bike. They claim the emissions were better than the current 4-stroke offerings. I realize that the dynamics with 2-cycles and 4-cycles are quite different, but I have to wonder why Honda hasn't been publicly pursuing HCCI technology in LDV's considering the experience they already have with motorcycles, and the fact that they seem to be lagging Toyota on the hybrid front. Honda has built fuel efficient vehicles since day one, and if I'm not mistaken, they still sell the most fuel efficient fleet in the US, but they seem to be conspicuously absent from the news these days with regard to fuel saving technology in passenger cars.


Honda's bringing 2.2 and 3.5 diesel engines to the U.S. in 2009/2010.

The 2.2, at least, looks to be plenty fuel-efficient:

>conspicuously absent from the news these days with regard to fuel saving technology in passenger cars.


GDI (with modestly higher compression ratio) can run lean and powerful due to stratified-charge arrangement. -- Roger Pham

With the same requirement for "costly" NOx treatment.

Remember that at a given compression ratio, Otto cycle (isochoric combustion) is significantly more efficient than Diesel cycle (isobaric combustion).

But not running stratified charge (where you would have all that excess air sucking up energy).

That said, a good development.


GDI stratified are having about the same particulate matter issues to be aftertreated as diesels have.
(hungry for HCCI news from GM &Co - might make it into those attractive small blocks)

Roger Pham

I'm sorry, cidi and mo, for bringing up the stratified charge business, for this is not the mode that this DiesOtto engine operates.
The article states that "all further emissions control in the DiesOtto engine is via a standard 3-way catalytic converter," meaning that GDI is used to spray fuel into the air during the compression stroke at stoichiometric ratio instead of lean-burn regime.
The efficiency gained in both HCCI and SI modes comes from engine down-sizing via turbocharging, thus allowing reduced pumping loss during low load limit operation.
The variable compression ability accounts for further increase efficiency in the high speed autobahn mode by raising compression at high engine speed or at low manifold pressure regime, and lowering the compression ratio at low engine speed but high manifold pressure regime in order to avoid engine knock.

Ethanol and water injection would be another way to permit higher compression ratio for improved efficiency at high engine speed.


Bud...We're on two different wavelengths. I talked of full EVs, not PHEVs. When EV technology develops much greater electric energy storage density, the days of ICE will end. Sooner the better, so that ICE isn't near humans anymore. Soon may ICE die, that life may live.

Bud Johns

Didn't make myself clear, litesong. PHEVs are right around the corner (actually here) whereas EVs are down the road a bit. Can't agree with you more that EVs are the ultimate solution. In the meantime, perfected ICEs are a big help.


ICE may be around for a while, take semi-trucks for example: batteries may be able to get cars around but trucks need to travel much further and drag much bigger loads.

Roger Pham

Considering the fact that ICE-HEV has comparable well-to-wheel efficiency as BEV using electricity from the most efficient power plants (combined-cycle gas-fired), the ICE will be around for a very long time...until you can replace most power plants today with wind, solar or nuclear energy...

...but wait, wind and solar energy are not reliable enough to replace fuel-burning power plants...and then, there are means available today to synthesize transportation fuels from wind and solar energy at very high efficiency level from source to wheel,almost comparable to solar or wind electricity to inverter to grid then to charger then to rectifier then to battery then to power inverter then to motor then to wheel.


Ideal type of engine to put into series hybrid, like planned GM Volt.


Although the term DiesOtto has been put forward as a catchy term for the operation it is timely to consider the fortunes of Diesel and Otto. Diesel suffered many problems in business and finally jumped overboard a ship to end his life. Otto had an equally tough time and died early in life. Ironically it was Gottlieb Daimler who rubbished his invention of what we call the Otto cycle engine. Daimler worked with Otto at Deutz and had a low opinon of Otto because he had no formal engineering education.

It was also Daimler who refused to accept the first reliable spark ignition system developed by Bosch, favoring a flame heated glowplug concept. Ref. "Vom Motor zum Auto" or "From Engines to Autos". It is therefore ironic that this DiesOtto named engine should come from a Daimler company.

By the way, no IC engines follow either the Diesel or Otto thermodynamic cycles. The closest theoretical cycle is the limited pressure cycle which is a combination of constant volume and constant pressure heat addition.

Rafael should consider that fuel enrichment at full load is required to protect the turbocharger in spark ingition engines. Diesel engines run lean even at full load and have substantially lower exhaust gas temperatures. They are also more efficient as a consequence. If that were not the case they would not be winning races like the Le Mans 24 hour would they?

Rafael Seidl

@ extracomment -

you're right, in most turbocharged engine designs, rated power is only possible with rich operation. However, two sequential turbos permit a setup in which the need for enrichment is substantially reduced - the low pressure turbine is further from the exhaust valves and the volute is larger. Both help reduce the turbine inlet temperature. In addition, the materials used in turbocharger turbines these days can support somewhat higher temps than those from a few years ago. Porsche even puts a VTG turbo in its 911 Turbo these days, though it isn't cheap.

It#s not clear from the press release which turbo system Mercedes' DiesOtto uses.

Bottom line: the DiesOtto is intended primarily to save fuel in part load operation. It needs a high power rating for Mercedes customers to even consider buying such a low-displacement engine in something as fancy as an S-Class. Those who really do frequently drive at very high speeds on the autobahn will still have he option of larger, more conventional engines.

Roger Pham


Thanks for the interesting bit of history. You've made a good point re. higher exhaust temp of gasoline engine, necessitating mixture enrichment for the turbocharge mode. But, turbocharged gasoline engine has low compression ratio around 8 or so in order to avoid detonation. This results in low expansion of combusted gas, hence high exhaust gas temp.

Now, what if you would raise the compression ratio in the high-speed turbocharged mode to 11 or higher, via the variable compression mechanism, instead, and use two-stage direct fuel injection instead of a single stage homogenous-charge-forming injection, in order to prevent detonaton. Inject half of the fuel early in the compression stroke, forming a mixture too lean for detonation, and then, inject the second half much later to help initiate ignition. There, no detonation, yet less PM given the partially homogenous charge. But, the higher compression ratio produces a much cooler exhaust due to higher expansion ratio for a given degree of peak combustion temperature. Your exhaust gas temp will still be higher than that of a Diesel, but hopefully low enough that you don't have to enrich your mixture, thereby wasting fuel, during the high-speed autobahn mode. Since you're closer to isochoric combustion than that of Diesel combustion, you can attain the efficiency of a Diesel with lower compression ratiio. Just a thought.

What do ya think?


Excuse for my English. Whether you know what variable compression ratio mechanism is applied in this engine

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